Retroreflectors in the form of sheeting are often used in highway signs and safety garments for highway construction workers to increase nighttime visibility. Retroreflective plastic plates are used as pavement markers and vehicle reflectors. Such retroreflective sheeting and plates typically comprise a layer of transparent plastic material having a substantially smooth front surface, and a rear surface provided with a plurality of retroreflective cube corner elements.
The cube corners in such products often achieve retroreflectivity by means of total internal reflection (TIR). Incident light beams entering the front surface of the article are reflected internally in prism-like fashion between the three surfaces of the cube corners and back out of the cube corners in the direction from which they came. Ideally, such retroreflectors should be able to retroreflect light beams entering the front surface of the sheeting not only at low entrance angles that are near zero (i.e. in a direction near normal to the sheeting) but at high entrance angles as well. Since the ability to internally reflect light at high entrance angles is dependent on the difference between the indices of refraction of the material forming the cube corners and the material that interfaces with the back surface of the cube corners, many retroreflective sheets and plates provide an air space behind the cube corners in order to maximize this difference. However, even when such an air backing is used, light entering the cube corners beyond a certain critical angle will begin to leak out of the cube corners by passing through one or more of the three faces of the cube corners.
To solve this problem and to increase the ability of the sheet to retroreflect entrance light at an even larger entrance angle, it is common to apply a reflective metallic layer such as vacuum-deposited aluminum to the rear surface of the cube corner elements. In such a structure, when light enters the cube corners, it is specularly reflected off of the metallic layers when it reaches the faces of the cube corners, and retroreflected back toward its source, even when entering at angles beyond the critical angle for TIR in an air-backed structure.
In addition to increasing the entrance angle for retroreflectivity, metallization provides a seal over the back surface of the cube corners that prevents TIR-destroying dirt and moisture from lodging on the back surfaces of the cube corners which would in turn degrade or destroy the ability of the cube corners to provide TIR. Metallization also allows the back surface to be fully supported (such as by the well-known “potting” technique) to give additional strength to the overall reflector construction.
Unfortunately, metallization has the disadvantage of reducing the overall retroreflectance of the article. Unlike TIR, for which reflectance is 100%, aluminum has a reflectance of only about 85%. Consequently, intensity of a retroreflected ray of light that is reflected off of three aluminized faces is reduced to about (85%)3 or about 61% of its corresponding TIR intensity.
Moreover, the provision of air spaces behind the cube corners to maximize the entrance angle for TIR can compromise the integrity of the resulting structure, rendering it mechanically weaker and thermally less stable than a solid construction. For some applications, such air spaces can be created by making the transparent material relatively thick and self-supporting, such as in vehicle taillight reflectors. However such structures require (relative to sheeting) large amounts of plastic material and hence are relatively expensive on a per-area basis. In applications where the transparent material must be thinner than tail light reflectors and yet stronger than sheeting, such as pavement marker reflectors, these air spaces can be created by ribs extending rearward of the cube corner elements to define “cells”, each cell containing one or more cube corner elements. While such ribs give strength to the retroreflector construction, and help to confine dirt or moisture-admitting damage to only those cube corner elements in the damaged cells, the area taken up by the ribs or cell walls is typically not retroreflective, and so decreases the overall retroflectivity of the article.
To overcome these shortcomings, retroreflective sheeting has been developed that employs, in lieu of air spaces, a solid backing layer having an index of refraction that is lower than the index of refraction of the transparent material used to form the cube corners. While such sheeting is structurally sounder and stronger than comparable sheeting employing air spaces behind the cube corners, the ability of such sheeting to provide TIR for a broad range of entrance angles is significantly less than that of air-backed articles because the difference in the index of refraction between the material forming the cube corners and the material forming the solid backing layer is less. For example, if the cube corners are formed from polycarbonate having an index of refraction n=1.59, and the solid backing material is cryolite having an index of refraction of n=1.32, the difference between the indices of refraction is 1.59−1.32=0.27. By contrast, when the backing layer is formed from air having an index of refraction n=1.00, the difference between the indices of refraction is 1.59−1.00=0.59 which is more than twice as much as 0.27. While this problem might be partially solved by the application of a reflective metallic layer over the back surface of the solid backing layer, such a retroreflector would rely even more upon the specular reflection provided by the reflective layer since the critical angle for TIR is smaller, which, as pointed out earlier, reduces the intensity of the retroreflected light by about 39%.
To increase the critical angle for TIR, materials having an index of refraction lower than cryolite have been used in such retroreflectors. For example, a thin optical film formed from particulate metal oxide such as silicon dioxide or alumina mixed with a binder has been applied as a backing layer to retroreflected sheeting. The resulting layer is characterized by nanoporosity and can have an index of refraction n as low as 1.10.